Which GOPer has the most at stake in Tuesday's debate?

Ben Carson responds to increased scrutiny

Republican candidates square off at 9 p.m.Tuesday night in Milwaukee on Fox Business Channel for their fourth debate, this time with just eight people on stage. There are huge stakes for Jeb Bush as the former Florida governor risks further alienating donors if he doesn't have a much stronger performance.

Bush has had some good days with his new, more open, freewheeling media strategy in New Hampshire and Iowa but he could kill it all with a bad debate performance. One Wall Streeter put it to me simply: "He has to stop the death spiral in his poll numbers."

Ben Carson will also likely face some more questions about claims in his autobiography that don't fully hold up to scrutiny. Fox moderators have suggested they will not try and repeat CNBC's aggressive approach, so Carson may be able to get away with just beating up on the media some more, always a popular approach with Republican audiences.

A worker helps prepare for the Republican presidential debate on November 9, 2015 in Milwaukee, Wisconsin.
Getty Images

Marco Rubio will get another shot to augment his rising status in the polls. There's no reason to expect he won't deliver another crisp performance that thrills many conservatives who view him as the strongest potential general election candidate to face Hillary Clinton.

But he will likely face increasing pressure over his immigration positions from Donald Trump and Ted Cruz. Trump needs to regain his footing with another breakout performance. And he should have a fairly friendly forum to do it. Trump can't really afford to vanish for long stretches as he has in the most recent debates.

Cruz is also well-positioned, especially if Carson and Trump drop out, so expect him to be very solicitous of the two front-runners.

Another big question is how hard Bush goes after Rubio. The Bush campaign clearly wants to torch Rubio but the donor class wants nothing to do with such an approach.

The question of Bush's approach to Rubio became a major issue on Tuesday after The New York Times reported that Bush's "Right to Rise" super PAC was contemplating a $20 million attack campaign criticizing Rubio as too pro-life to win a general election campaign.

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The threat is deeply strange for a number of reasons. Hedge fund manager Anthony Scaramucci is quoted in the Times piece cautioning against a scorched earth approach to Rubio. There are many more big Wall Street Bush backers who are vehemently opposed to this approach, including some for whom it would be a final straw to drive them to Rubio.

One of Bush's main Wall Street supporters recently told me he was furious after Bush's attack on Rubio in the last debate and that he had strongly cautioned the former Florida governor against such an approach.

There is also an open question about what ripping down Rubio would really do for Bush. Even if he inherited all of Rubio's current supporters, Bush would still be at only around 18 percent in the polls, well behind Trump and Carson. The theory is that Trump and Carson will eventually fade, leaving Bush, Rubio and Ted Cruz (and maybe others) to fight over their supporters. There is also the deep weirdness of going after Rubio in a GOP primary for being too pro-life. This sounds more like an in-kind contribution to the Rubio campaign than an attack.

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Rubio's campaign wasted no time seizing on the Times report Monday night. In a blast email headlined "This is a Joyful Campaign?," Rubio campaign manager Terry Sullivan wrote: "You read that right. Jeb's plan is to spend $20 million dollars to 'damage Mr. Rubio's reputation.' Not just talk about how they differ on the issues but to actively try to destroy Marco's reputation. And they're bragging about that? How is that the kind of 'joyful' campaign that Jeb claimed he wanted to run?!?! … Our campaign is preparing to fight back, but without your immediate support with a donation of $25 or more by clicking right here right now it won't be sufficient."

Bush is not going to win the Republican nomination by destroying Rubio. He is more likely to only turn off donors and possibly engender greater sympathy for the Florida senator if he goes this route.

And Bush's heart is not in it. That was obvious when Rubio easily swatted back Bush's attacks in the last debate. If Bush tries it again Tuesday night, he can likely expect more of the same.

— Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.